The present invention relates to a positioning device, in particular a device having at least one element capable of occupying different predetermined positions between two extreme positions. The element being coupled to a driving shaft driven by a stepping motor.
Devices of this kind often comprise different mechanical elements, some of which are mounted with axial play not exceeding a previously assigned limit in order to make the device capable of operating in conformity with required criteria of precision and reliability. Consequently, the present invention also relates to a method of adjustment of such axial play.
Such devices can be used in different areas requiring the same kind of technical application. A preferred application is in the positioning of the optics or headlamps of a car.
The positioning of optics of a car can be done in different ways. Frequently a direct current or DC motor is used. A potentiometer transmits the position of the driving shaft to a control device (which in this application is axially displaceable), that is an indication of the position of the shaft at a given time. The maintenance of this position is executed as a function of a given signal. It concerns, therefore, a control of the analog type working in a close-loop.
It is, however, observed that drivers using a DC motor are not satisfactory for the following reasons.
First, with known devices, a problem of compatibility is encountered because of the increasing use of digital controls. It is desirable that the positioning device be connected to a microprocessor, which is not feasible in the case of a DC motor.
Second, it is observed that the potentiometer is exposed to wear, and change in its operation is accelerated as a result of the environmental conditions of use, such as a high temperature range. For these reasons, the accuracy of positioning is not ensured in spite of mechanical gearing down provided on most of the known devices.
Patent application EP-A-0 340 735 discloses a device for the adjustment of the inclination of a headlamp of a car, that is, of the light beam. This adjustment requires a stepping motor having a direction of rotation and number of turns dependent on a microprocessor, and a control unit for the signals received from an optical sensor having five elements E1 to E5. However, the patent does not disclose or suggest any means for repositioning the optical beam, nor for taking up the axial play.
Patent application EP 0 182 774 describes the problem of correcting the position of the element to be adjusted; a correction necessary because of the loss of steps of the stepping motor. The solution proposed in this case consists of momentarily delivering a determined higher current to the motor, if the number of steps to be executed and the number of effectively realized steps are different.
This solution presents, among others, the drawback of creating an excessive stress on the stepping motor as a function of the extent of the obstacle to overcome, which renders the correction necessary. Moreover, this approach generates disturbances in the stepping motor so that the desired position may not be attained or attained with such error margin that the system is unsuitable.
Thus, the problem of the reliability of the positioning devices, and above all of the precision of their operation, still remains unresolved. It is fundamental, however, from the moment the motor is switched on, to bring the headlamp of a car from position "1" to position "2", the latter should be effectively reached by the headlamp with a tolerance as small as possible and independent of the environmental conditions of use. Therefore, in a preferred device, the absolute error in positioning of the headlamps of a car should not exceed 0.1 mm. Moreover, it is necessary that the position attained be maintained in spite of external stress in order to fully benefit from the advantages of the relatively precise positioning. The solution to these problems is dependent on the precision which can be attained in the axial play, which should not exceed 0.05 mm, for the field of application of the present invention.